1. Field of the Invention
This invention relates to windmills, that is to say apparatus that include a wind driven rotor and are used to convert the kinetic energy of the wind into useful work or other forms of energy.
More particularly, the invention relates to such apparatus wherein the rotor is incorporated in a kite, that is to say a tethered flying platform that may be supported entirely by its interaction with a wind of appropriate strength. Apparatus of that kind is referred to as a windmill kite herein.
2. Prior Art
A windmill kite comprising a high flying platform incorporating two wind driven rotors, two dynamos mounted on the platform respectively driven by the rotors, three tensile tethering lines extending from the platform to ground level anchorages and conductive cables connecting the dynamos to a ground-based electricity transmission system or other electric load, has been previously proposed by the present inventor and others.
The term “dynamo is used in the preceding paragraph and hereinafter in its generic sense as a machine for converting mechanical energy into electrical energy, or vice versa. The term designates a reversible machine; any dynamo may be used either as a generator or as a motor. Moreover, the term covers both AC and DC machines.
The purpose of incorporating the wind driven rotors in a kite is to enable them to be positioned at a high altitude where relatively strong and continuous winds may be expected to be found.
Each wind driven rotor (referred to simply as “a mill rotor” hereinafter) comprises a rotatable hub and a plurality of equi-angularly spaced blades extending radially from the hub. Preferably each blade is of aerofoil section and blade pitch control means are provided whereby the angle of attack of the blades may be adjusted from time to time.
Mill rotors are also reversible machines. On the one hand, wind directed through the swept area of the blades induces a continuous rotor torque, enabling the mill rotor to, for example, drive an associated dynamo as a generator. On the other hand, rotation of the mill rotor in still air by a dynamo acting as a motor induces a continuous air flow through the swept area producing a thrust force, enabling the mill rotor to, for example, lift itself and the dynamo from the ground.
That previously proposed windmill kite was described and evaluated in a paper entitled “Flying Electric Generator to Harness Jetstream Energy” delivered by the present inventor to the “Space 2000” Conference of the American Society of Civil Engineers held at Albuquerque USA in February–March 2000, and is presently available in the published proceedings of that Society at pp 1020–6.
The windmill kite described in that paper comprised an elongate flying platform including two outrigger mill rotors disposed symmetrically on each side of the longitudinal centre line of the platform. Each mill rotor was connected by a geared drive-transmission connecting it to a dynamo carried by the platform.
The flying platform, as illustrated in that paper, has a leading end and a trailing end relative to the direction of the wind, with adjustable but essentially fixed, preferably aerofoil, control surfaces at the ends of the platform. Those control surfaces are intended to ensure that the leading end of the platform is directed into the wind and that a transverse axis extending through the centers of the mill rotors remains substantially horizontal. Those control surfaces are also intended to ensure that the relative elevations of the ends of the platform are maintained so as to produce a substantially constant pitch angle for the platform as a whole, at which the wind flows obliquely against the undersides of the mill rotors.
As described in that paper, under those circumstances a drag force acts upon the mill rotors and platform, and a thrust force having an upward component and a torque are developed by each mill rotor. The upward component of the thrust forces and drag forces are opposed by the weight of the platform and the items carried by it and by tension in the tethering lines extending from the platform to ground level anchorages.
Those anchorages include a winch or winches that enable the flying height of the platform to be adjusted as needed.
The torque reactions from the mill rotors acting on the platform are cancelled out by ensuring that the mill rotors rotate in opposite directions. The generated torque is used to drive the dynamo or dynamos when operating in the generator mode.
A major problem with that prior proposal arises from the fact that, even at high altitudes, winds sometimes fail to blow with sufficient strength to enable those control surfaces to adequately stabilize the flying platform against variables such as wind gusts or eddies. Thus, should the wind fail, it has been necessary for the platform to be winched down to prevent the tethering lines and/or conductive cables from becoming tangled, and, in a worst case scenario, to prevent the platform from crashing. Winching the platform down and subsequently returning it to an operating altitude is a time consuming and expensive operation.